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SubjectRe: [RFD] Layering: Use-Case Composers (was: DRBD - what is it, anyways? [compare with e.g. NBD + MD raid])
Evgeniy Polyakov wrote:
> Al Boldi ( wrote:
> > Look at ZFS; it illegally violates layering by combining md/dm/lvm with
> > the fs, but it does this based on a realistic understanding of the
> > problems involved, which enables it to improve performance, flexibility,
> > and functionality specific to its use case.
> >
> > This implies that there are two distinct forces at work here:
> >
> > 1. Layer components
> > 2. Use-Case composers
> >
> > Layer components should technically not implement any use case (other
> > than providing a plumbing framework), as that would incur unnecessary
> > dependencies, which could reduce its generality and thus reusability.
> >
> > Use-Case composers can now leverage layer components from across the
> > layering hierarchy, to yield a specific use case implementation.
> >
> > DRBD is such a Use-Case composer, as is mdm / dm / lvm and any fs in
> > general, whereas aoe / nbd / loop and the VFS / FUSE are examples of
> > layer components.
> >
> > It follows that Use-case composers, like DRBD, need common functionality
> > that should be factored out into layer components, and then recompose to
> > implement a specific use case.
> Out of curiosity, did you try ndb+dm+raid1 compared to drbd and/or zfs
> on top of distributed storage (which is a urprise to me, that holy zfs
> suppors that)?

Actually, I may not have been very clear in my Use-Case composer description
to mean internal in-kernel Use-Case composer as opposed to external Userland
Use-Case composer.

So, nbd+dm+raid1 would be an external Userland Use-Case composition, which
obviously could have some drastic performance issues.

DRBD and ZFS are examples of internal in-kernel Use-Case composers, which
obviously could show some drastic performance improvements.

Although you could allow in-kernel Use-Case composers to be run on top of
Userland Use-Case composers, that wouldn't be the preferred mode of
operation. Instead, you would for example recompose ZFS to incorporate an
in-kernel distributed storage layer component, like nbd.

All this boils down to refactoring Use-Case composers to produce layer
components with both in-kernel and userland interfaces. Once we have that,
it becomes a matter of plug-and-play to produce something awesome like ZFS.



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